US 20090073444 A1
This application describes designs, implementations, and techniques for controlling propagation mode or modes of light in a common optical path, which may include one or more waveguides, to sense a sample.
1. A device for optically measuring a sample, comprising:
a light source to produce an input beam for optically probing a sample;
a waveguide having a proximal end to receive the input beam from the light source and a distal end towards which the received input beam is guided by the waveguide;
a probe head coupled to the distal end of the waveguide to receive the input beam and to reflect a first portion of the input beam back to the waveguide and direct a second portion of the input beam to the sample, the probe head configured to overlap reflection of the second portion from the sample with the first portion and to export to the waveguide the reflection as a reflected second portion;
a differential delay modulator in optical communication with the proximate end of the waveguide to receive light in the first portion and the reflected second portion from the proximate end of the waveguide, the differential delay modulator operable to split the received light into a first beam and a second beam and to produce variable relative phase delays between the first beam and the second beam; and
a detection module to detect light that combines the first beam and the second beam and is output by the differential delay modulator, the detection module operable to extract information of the sample carried by the reflected second portion at different depths in the sample based the variable relative phase delays produced by the differential delay modulator.
This application is a continuation application of co-pending U.S. patent application Ser. No. 11/842,913, filed Aug. 21, 2007; which is a continuation application of U.S. patent application Ser. No. 11/200,498, filed Aug. 8, 2005, now U.S. Pat. No. 7,259,851; which is a continuation application of U.S. patent application Ser. No. 10/860,094, filed Jun. 3, 2004, now U.S. Pat. No. 6,943,881; which claims the benefit of the following four U.S. Provisional Applications:
1. Ser. No. 60/475,673 entitled “Method and Apparatus for Acquiring Images of Optical Inhomogeneity in Substances” and filed Jun. 4, 2003;
2. Ser. No. 60/514,768 entitled “Coherence-Gated Optical Glucose Monitor” and filed Oct. 27, 2003;
3. Ser. No. 60/526,935 entitled “Method and Apparatus for Acquiring Images of Optical Inhomogeneity in Substances” and filed Dec. 4, 2003; and
4. Ser. No. 60/561,588 entitled “Acquiring Information of Optical Inhomogeneity and Other Properties in Substances” and filed Apr. 12, 2004.
The entire disclosures of the above-referenced applications are incorporated herein by reference as part of this application.
This application relates to non-invasive, optical probing of various substances, including but not limited to, skins, body tissues and organs of humans and animals.
Investigation of substances by non-invasive and optical means has been the object of many studies as inhomogeneity of light-matter interactions in substances can reveal their structural, compositional, physiological and biological information. Various devices and techniques based on optical coherence domain reflectometry (OCDR) may be used for non-invasive optical probing of various substances, including but not limited to skins, body tissues and organs of humans and animals, to provide tomographic measurements of these substances.
In many OCDR systems, the light from a light source is split into a sampling beam and a reference beam which propagate in two separate optical paths, respectively. The light source may be partially coherent source. The sampling beam is directed along its own optical path to impinge on the substances under study, or sample, while the reference beam is directed in a separate path towards a reference surface. The beams reflected from the sample and from the reference surface are then brought to overlap with each other to optically interfere. Because of the wavelength-dependent phase delay the interference results in no observable interference fringes unless the two optical path lengths of the sampling and reference beams are very similar. This provides a physical mechanism for ranging. A beam splitter may be used to split the light from the light source and to combine the reflected sampling beam and the reflected reference beam for detection at an optical detector. This use of the same device for both splitting and recombining the radiation is essentially based on the well-known Michelson interferometer. The discoveries and the theories of the interference of partially coherent light are summarized by Born and Wolf in “Principles of Optics”, Pergamon Press (1980).
Low-coherence light in free-space Michelson interferometers were utilized for measurement purposes. Optical interferometers based on fiber-optic components were used in various instruments that use low-coherence light as means of characterizing substances. Various embodiments of the fiber-optic OCDR exist such as devices disclosed by Sorin et al in U.S. Pat. No. 5,202,745, by Marcus et al in U.S. Pat. No. 5,659,392, by Mandella et al in U.S. Pat. No. 6,252,666, and by Tearney et al in U.S. Pat. No. 6,421,164. The application of OCDR in medical diagnoses in certain optical configurations has come to known as “optical coherence tomography” (OCT).
The designs, techniques and exemplary implementations for non-invasive optical probing described in this application use the superposition and interplay of different optical waves and modes propagating along substantially the same optical path inside one or more common optical waveguides. When one of the optical waves or modes interacts with the substance under study its superposition with another wave or mode can be used for the purpose of acquiring information about the optical properties of the substance.
The methods and apparatus described in this application are at least in part based on the recognition of various technical issues and practical considerations in implementing OCDR in commercially practical and user friendly apparatus, and various technical limitations in OCDR systems disclosed by the above referenced patents and other publications. As an example, at least one of disadvantages associated to the OCDR system designs shown in
In various examples described in this application, optical radiation is not physically separated to travel different optical paths. Instead, all propagation waves and modes are guided along essentially the same optical path through one or more common optical waveguides. Such designs with the common optical path may be advantageously used to stabilize the relative phase among different radiation waves and modes in the presence of environmental fluctuations in the system such as variations in temperatures, physical movements of the system especially of the waveguides, and vibrations and acoustic impacts to the waveguides and system. In this and other aspects, the present systems are designed to do away with the two-beam-path configurations in various interferometer-based systems in which sample light and reference light travel in different optical paths in part to significantly reduce the above fluctuation and drift in the differential phase delay. Therefore, the present systems have a “built-in” stability of the differential optical path by virtue of their optical designs and are beneficial for some phase-sensitive measurement, such as the determination of the absolute reflection phase and birefringence. In addition, the techniques and devices described in this application simplify the structures and the optical configurations of devices for optical probing by using the common optical path to guide light.
In various applications, it may be beneficial to acquire the absorption characteristics of the material in an isolated volume inside the sample. In other case it may be desirable to map the distribution of some substances identifiable through their characteristic spectral absorbance. In some OCDR systems such as systems in aforementioned patents, it may be difficult to perform direct measurements of the optical inhomogeneity with regard to these and other spectral characteristics. The systems and techniques described in this application may be configured to allow for direct measurements of these and other spectral characteristics of a sample.
Exemplary implementations are described below to illustrate various features and advantages of the systems and techniques. One of such features is methods and apparatus for acquiring information regarding optical inhomogeneity in substance by a non-invasive means with the help of a low-coherence radiation. Another feature is to achieve high signal stability and high signal-to-noise ratio by eliminating the need of splitting the light radiation into a sample path and a reference path. Additional features include, for example, a platform on which phase-resolved measurements such as birefringence and absolute refractive indices can be made, capability of acquiring optical inhomogeneity with regard to the spectral absorbance, solving the problem of signal drifting and fading caused by the polarization variation in various interferometer-based optical systems, and an effective use of the source radiation with simple optical arrangements. Advantages of the systems and techniques described here include, among others, enhanced performance and apparatus reliability, simplified operation and maintenance, simplified optical layout, reduced apparatus complexity, reduced manufacturing complexity and cost.
Various exemplary methods and techniques for optically sensing samples are described. For example, one method for optically measuring a sample includes the following steps. A beam of guided light in a first propagation mode is directed to a sample. A first portion of the guided light in the first propagation mode is directed away from the sample at a location near the sample before the first portion reaches the sample. A second portion in the first propagation mode is directed to reach the sample. A reflection of the second portion from the sample is controlled to be in a second propagation mode different from the first propagation mode to produce a reflected second portion. Both the reflected first portion in the first propagation mode and the reflected second portion in the second propagation mode are then directed through a common waveguide into a detection module to extract information from the reflected second portion on the sample.
Another method for optically measuring a sample is also described. In this method, light in a first propagation mode is directed to a vicinity of a sample under measurement. A first portion of the light in the first propagation mode is then directed to propagate away from the sample at the vicinity of the sample without reaching the sample. A second portion of the light in the first propagation mode is directed to the sample to cause reflection at the sample. The reflected light from the sample is controlled to be in a second propagation mode that is independent from the first propagation mode to co-propagate with the first portion along a common optical path. The first portion in the first propagation mode and the reflected light in the second propagation mode are used to obtain information of the sample.
This application further describes exemplary implementations of devices and systems for optically measuring samples. One example of such devices includes a waveguide to receive and guide an input beam in a first propagation mode, and a probe head coupled to the waveguide to receive the input beam and to reflect a first portion of the input beam back to the waveguide in the first propagation mode and direct a second portion of the input beam to a sample. This probe head collects reflection of the second portion from the sample and exports to the waveguide the reflection as a reflected second portion in a second propagation mode different from the first propagation mode. This device further includes a detection module to receive the reflected first portion and the reflected second portion in the waveguide and to extract information of the sample carried by the reflected second portion.
In another example, an apparatus for optically measuring a sample is disclosed to include a light source, a waveguide supporting at least a first and a second independent propagation modes and guiding the light radiation from the light source in the first propagation mode to the vicinity of a sample under examination, a probe head that terminates the waveguide in the vicinity of the sample and reverses the propagation direction of a portion of the first propagation mode in the waveguide while transmitting the remainder of the light radiation to the sample, the probe head operable to convert reflected light from the sample into the second propagation mode, and a differential delay modulator that transmits the light in both the first and the second propagation modes from the probe head and the waveguide and varies the relative optical path length between the first and the second propagation modes. In this apparatus, a mode combiner is included to receive light from the differential delay modulator and operable to superpose the first and the second propagation modes by converting a portion of each mode to a pair of new modes. At least one photodetector is used in this apparatus to receive light in at least one of the two new modes. Furthermore, an electronic controller is used in communication with the photodetector and is operable to extract information of the sample from the output of the photodetector.
In yet another example, a device is described to include an optical waveguide, an optical probe head and an optical detection module. The optical waveguide is to guide an optical radiation in a first optical mode. The optical probe head is coupled to the optical waveguide to receive the optical radiation. The optical probe head is operable to (1) redirect a portion of the optical radiation back to the optical waveguide while transmitting the remaining radiation to a sample, (2) receive and direct the reflected or backscattered radiation from the sample into the waveguide, and (3) control the reflected or the backscattered light from the sample to be in a second optical mode different from the first optical mode. The optical detection module is used to receive the radiation redirected by the probe head through the waveguide and to convert optical radiation in the first and second optical modes, at least in part, into a common optical mode.
A further example for a device for optically measuring a sample includes an input waveguide, an output waveguide and a probe head. The input waveguide supports a first and a second different propagation modes and is used to receive and guide an input beam in the first propagation mode. The output waveguide supports a first and a second different propagation modes. The probe head is coupled to the input waveguide to receive the input beam and to the output waveguide to export light. The probe head is operable to direct a first portion of the input beam in the first propagation mode into the output waveguide and direct a second portion of the input beam to a sample. In addition, the probe head collects reflection of the second portion from the sample and exports to the output waveguide the reflection as a reflected second portion in the second propagation mode. Furthermore, this device includes a detection module to receive the reflected first portion and the reflected second portion in the output waveguide and to extract information of the sample carried by the reflected second portion.
This application also describes an example of an apparatus for optically measuring a sample. In this example, a first waveguide capable of maintaining at least one propagation mode is used. A light source that emits radiation is used to excite the propagation mode in the first waveguide. A light director is used to terminate the first waveguide with its first port, to pass the light mode entering the first port, at least in part, through a second port, and to pass the light modes entering the second port, at least in part, through a third port. The apparatus also includes a second waveguide that supports at least two independent propagation modes and having a first end coupled to the second port and a second end. Notably, a probe head is coupled to the second end of the second waveguide and operable to reverse the propagation direction of the light in part back to the second waveguide and to transmit the remainder to the sample. This probe head is operable to transform the collected light from the sample reflection to an orthogonal mode supported by the second waveguide and direct light in the orthogonal mode into the second waveguide. A third waveguide is also included which supports at least two independent propagation modes and is connected to the third port of the light director to receive light therefrom. A differential delay modulator is used to connect to the third waveguide to receive light from the second waveguide and imposes a variable phase delay and a variable path length on one mode in reference to the other. A fourth waveguide supporting at least two independent modes is coupled to the differential delay modulator to receive light therefrom. A detection subsystem is positioned to receive light from the fourth waveguide and to superpose the two propagation modes from the fourth waveguide to form two new modes, mutually orthogonal. This detection subsystem includes two photo-detectors respectively receiving light in the new modes.
These and other features, system configurations, associated advantages, and implementation variations are described in detail in the attached drawings, the textual description, and the claims.
Energy in light traveling in an optical path such as an optical waveguide may be in different propagation modes. Different propagation modes may be in various forms. States of optical polarization of light are examples of such propagation modes. Two independent propagation modes do not mix with one another in the absence of a coupling mechanism. As an example, two orthogonally polarization modes do not interact with each other even though the two modes propagate along the same optical path or waveguide and are spatially overlap with each other. The exemplary techniques and devices described in this application use two independent propagation modes in light in the same optical path or waveguide to measure optical properties of a sample. A probe head may be used to direct the light to the sample, either in two propagation modes or in a single propagation modes, and receive the reflected or back-scattered light from the sample.
For example, one beam of guided light in a first propagation mode may be directed to a sample. A first portion of the first propagation mode may be arranged to be reflected before reaching the sample while the a second portion in the first propagation mode is allowed to reach the sample. The reflection of the second portion from the sample is controlled in a second propagation mode different from the first propagation mode to produce a reflected second portion. Both the reflected first portion in the first propagation mode and the reflected second portion in the second propagation mode are directed through a common waveguide into a detection module to extract information from the reflected second portion on the sample.
In another example, optical radiation in both a first propagation mode and a second, different propagation mode may be guided through an optical waveguide towards a sample. The radiation in the first propagation mode is directed away from the sample without reaching the sample. The radiation in the second propagation mode is directed to interact with the sample to produce returned radiation from the interaction. Both the returned radiation in the second propagation mode and the radiation in the first propagation mode are coupled into the optical waveguide away from the sample. The returned radiation in the second propagation mode and the radiation in the first propagation mode from the optical waveguide are then used to extract information of the sample.
In these and other implementations based on the disclosure of this application, two independent modes are confined to travel in the same waveguides or the same optical path in free space except for the extra distance traveled by the probing light between the probe head and the sample. This feature stabilizes the relative phase, or differential optical path, between the two modes of light, even in the presence of mechanical movement of the waveguides. This is in contrast to interferometer sensing devices in which sample light and reference light travel in different optical paths. These interferometer sensing devices with separate optical paths are prone to noise caused by the variation in the differential optical path, generally complex in optical configurations, and difficult to operate and implement. The examples described below based on waveguides are in part designed to overcome these and other limitations.
The superposition of the two modes 001 and 002 in the detection subsystem 260 allows for a range detection. The light entering the detection subsystem 260 in the mode 002 is reflected by the sample, bearing information about the optical inhomogeneity of the sample 205, while the other mode, 001, bypassing the sample 205 inside probe head 220. So long as these two modes 001 and 002 remain independent through the waveguides their superposition in the detection subsystem 260 may be used to obtain information about the sample 205 without the separate optical paths used in some conventional Michelson interferometer systems.
For the simplicity of the analysis, consider a thin slice of the source spectrum by assuming that the amplitude of the mode 001 is E001 in a first linear polarization and that of the mode 002 is E002 in a second, orthogonal linear polarization in the first waveguide 271. The sample 205 can be characterized by an effective reflection coefficient r that is complex in nature; the differential delay modulator 350 can be characterized by a pure phase shift F exerted on the mode 001. Let us now superpose the two modes 001 and 002 by projecting them onto a pair of new modes, EA and EB, by a relative 45-degree rotation in the vector space. The new modes, EA and EB, may be expressed as following:
It is assumed that all components in the system, except for the sample 205, are lossless. The resultant intensities of the two superposed modes are
where φ is the phase delay associated with the reflection from the sample. A convenient way to characterize the reflection coefficient r is to measure the difference of the above two intensities, i.e.
If Γ is modulated by the differential delay modulator 250, the measured signal, Eq. (3), is modulated accordingly. For either a periodic or a time-linear variation of Γ, the measured responds with a periodic oscillation and its peak-to-peak value is proportional to the absolute value of r.
For a broadband light source 201 in
Due to the stability of the relative phase between the two modes, 001 and 002, phase-sensitive measurements can be performed with the system in
In this method, a sinusoidal modulation is applied to the differential phase by the differential delay modulator 250, with a modulation magnitude of M and a modulation frequency of Q. The difference in intensity of the two new modes is the measured and can be expressed as follows:
It is clear from Eq. (4) that the measured exhibits an oscillation at a base frequency of Ω and oscillations at harmonic frequencies of the base frequency Ω. The amplitudes of the base frequency and each of the harmonics are related to φ and |r|. The relationships between r and the harmonics can be derived. For instance, the amplitude of the base-frequency oscillation and the second harmonic can be found from Eq. (4) to be:
where J1 and J2 are Bessel functions of the first and second order, respectively. Eq. (5a) and (5b) can be used to solve for |r| and φ, i.e. the complete characterization of r. We can therefore completely characterize the complex reflection coefficient r by analyzing the harmonic content of various orders in the measured signal. In particular, the presence of the base-frequency component in the measured is due to the presence of φ.
The system in
In the illustrated implementation, the probe head 320 includes a lens system 321 and a polarization-selective reflector (PSR) 322. The lens system 321 is to concentrate the light energy into a small area, facilitating spatially resolved studies of the sample in a lateral direction. The polarization-selective reflector 322 reflects the mode 001 back and transmits the mode 002. Hence, the light in the mode 002 transmits through the probe head 320 to impinge on the sample 205. Back reflected or scattered the light from the sample 205 is collected by the lens system 321 to propagate towards the circulator 310 along with the light in the mode 001 reflected by PSR 322 in the waveguide 372.
In the implementation illustrated in
The system in
For acquiring two-dimensional images of optical inhomogeneity in the sample 205, the probe head 320 may be controlled via a position scanner such as a translation stage or a piezo-electric positioner so that the probing light scans in a lateral direction, perpendicular to the light propagation direction. For every increment of the lateral scan a profile of reflection as a function of depth can be recorded with the method described above. The collected information can then be displayed on a display and interface module 372 to form a cross-sectional image that reveals the inhomogeneity of the sample 205.
In general, a lateral scanning mechanism may be implemented in each device described in this application to change the relative lateral position of the optical probe head and the sample to obtain a 2-dimensional map of the sample. A xy-scanner, for example, may be engaged either to the optical head or to a sample holder that holds the sample to effectuate this scanning in response to a position control signal generated from the electronic controller 370.
The light from the source 201 is typically partially polarized. The polarizer 510 may be aligned so that maximum amount of light from the source 201 is transmitted and that the transmitted light is coupled to both of the guided modes in the waveguide 271 with the substantially equal amplitudes. The electric fields for the two orthogonal polarization modes S and P in the waveguide 271 can be expressed as:
where the electric field transmitting the polarizer is denoted as E. It should be appreciated that the light has a finite spectral width (broadband or partially coherent). The fields can be described by the following Fourier integral:
For the simplicity of the analysis, a thin slice of the spectrum, i.e. a lightwave of a specific wavelength, is considered below. Without loosing generality, it is assumed that all the components, including polarizers, waveguides, Router, PSR and VDGD, are lossless. Let us designate the reflection coefficient of the sample r, that is complex in nature. The p-wave picks up an optical phase, Γ, relative to the s-wave as they reach the second polarizer 540:
The light that passes through Polarizer 540 can be expressed by
The intensity of the light that impinges on the photodetector 550 is given by:
where phase angle δ reflects the complex nature of the reflection coefficient of the sample 205 and is defined by
Assuming the modulator 520 exerts a sinusoidal phase modulation, with magnitude M and frequency Ω, in the p-wave with respect to the s-wave, the light intensity received by the detector 550 can be expressed as follows:
where phase angle φ is the accumulated phase slip between the two modes, not including the periodic modulation due to the modulator 520. The VDGD 530 or a static phase shift in the modulator 520, may be used to adjust the phase difference between the two modes to eliminate φ. The waveform of I is graphically shown in
where J1 and J2 are Bessel functions of the first and second order, respectively. Eq. (13a) and (13b) can be used to solve for |r| and δ, i.e. the complete characterization of r.
The effect of having a broadband light source 201 in the system in
It is easy to see that if the range of φ(λ) is comparable to π for the bandwidth of the light source no oscillation in I can be observed as oscillations for different wavelengths cancel out because of their phase difference. This phenomenon is in close analogy to the interference of white light wherein color fringes are visible only when the path difference is small (the film is thin). The above analysis demonstrates that the use of a broadband light source enables range detection using the proposed apparatus. In order to do so, let the s-wave to have a longer optical path in the system compared to the p-wave (not including its round-trip between Probing Head and Sample). For any given path length difference in the system there is a matching distance between Probing Head and Sample, z, that cancels out the path length difference. If an oscillation in I is observed the p-wave must be reflected from this specific distance z. By varying the path length difference in the system and record the oscillation waveforms we can therefore acquire the reflection coefficient r as a function of the longitudinal distance z, or depth. By moving Probing Head laterally, we can also record the variation of r in the lateral directions.
In the above implementations, light for sensing the sample 205 is not separated into two parts that travel along two different optical paths. Two independent propagation modes of the light are guided essentially in the same waveguide at every location along the optical path except for the extra distance traveled by one mode between the probe head 320 and the sample 205. After redirected by the probe head 320, the two modes are continuously guided in the same waveguide at every location along the optical path to the detection module.
Alternatively, the light from the light source to the probe head may be controlled in a single propagation mode (e.g., a first propagation mode) rather than two different modes. The probe head may be designed to cause a first portion of the first mode to reverse its propagation direction while directing the remaining portion, or a second portion, to reach the sample. The reflection or back scattered light of the second portion from the sample is collected by the probe head and is controlled in the second propagation mode different from the first mode to produce a reflected second portion. Both the reflected first portion in the first propagation mode and the reflected second portion in the second propagation mode are directed by the probe head through a common waveguide into the detection module for processing. In comparison with the implementations that use light in two modes throughout the system, this alternative design further improves the stability of the relative phase delay between the two modes at the detection module and provides additional implementation benefits.
The probe head 820 is designed differently from the prove head 320 in that the probe head 830 converts part of light in the mode 001 into the other different mode 002 when the light is reflected or scattered back from the sample 205. Alternatively, if the light in the waveguide 272 that is coupled from the waveguide 871 is in the mode 002, the probe head 820 converts that part of light in the mode 002 into the other different mode 001 when the light is reflected or scattered back from the sample 205. In the illustrated example, the probe head 820 performs these functions: a) to reverse the propagation direction of a small portion of the incoming radiation in mode 001; b) to reshape the remaining radiation and transmit it to the sample 205; and c) to convert the radiation reflected from the sample 205 to an independent mode 002 supported by the dual-mode waveguide 272. Since the probe head 820 only converts part of the light into the other mode supported by the waveguide 272, the probe head 820 is a partial mode converter in this regard. Due to the operations of the probe head 820, there are two modes propagating away from the probe head 820, the mode 001 that bypasses the sample 205 and the mode 002 for light that originates from sample reflection or back scattering. From this point on, the structure and operations of the rest of the system shown in
The probe head 820 in
In the examples in
A number of hardware choices are available for differential delay modulator 250.
In one implementation, a non-mechanical design may include one or more segments of tunable birefringent materials such as liquid crystal materials or electro-optic birefringent materials such as lithium niobate crystals in conjunction with one or more fixed birefringent materials such as quartz and rutile. The fixed birefringent material provides a fixed delay between two modes and the tunable birefringent material provides the tuning and modulation functions in the relative delay between the two modes.
The variable delay element in one of the two optical paths may be implemented in various configurations. For example, the variable delay element may be a mechanical element. A mechanical implementation of the device in
The mechanical delay device shown in
In this device, the variation of the optical path length is caused by the rotation of the Optical Plate 1320. The Optical Plate 1320 may be made of a good quality optical material. The two optical surfaces may be flat and well polished to minimize distortion to the light beam. In addition, the two surfaces should be parallel to each other so that the light propagation directions on both sides of the Optical Plate 1320 are parallel. The thickness of the Optical Plate 1320 may be chosen according to the desirable delay variation and the range of the rotation angle. The optical path length experienced by the light beam is determined by the rotation angle of the Optical Plate 1320. When the surfaces of the Optical Plate 1320 is perpendicular to the light beam (incident angle is zero), the path length is at its minimum. The path length increases as the incident angle increases.
If a linearly polarized light is used as the input beam 1300 in
The beam splitter 1310 used in
In the above examples, a single dual-mode waveguide 272 or 372 is used as an input and output waveguide for the probe head 220, 320, or 820. Hence, the input light, either in a single mode or two independent modes, is directed into the probe head through that dual-mode waveguide 272 or 372, and the output light in the two independent modes is also directed from the probe head to the detection subsystem or detector.
Alternatively, the single dual-mode waveguide 272 or 372 may be replaced by two separate waveguides, one to direct input light from the light source to the probe head and another to direct light from the probe head to the detection subsystem or detector. As an example, the device in
The above-described devices and techniques may be used to obtain optical measurements of a given location of the sample at different depths by controlling the relative phase delay between two modes at different values and optical measurements of different locations of the sample to get a tomographic map of the sample at a given depth or various depths by laterally changing the relative position of the probe head over the sample. Such devices and techniques may be further used to perform other measurements on a sample, including spectral selective measurements on a layer of a sample.
In various applications, it may be beneficial to obtain information about certain substances, identifiable through their spectral absorbance, dispersed in the samples. For this purpose, a tunable bandpass filter may be used to either filter the light incident to the probe head to select a desired spectral window within the broadband spectrum of the incident light to measure the response of the sample and to vary the center wavelength of the spectral window to measure a spectral distribution of the responses of the sample. This tuning of the bandpass filter allows a variable portion of the source spectrum to pass while measuring the distribution of the complex reflection coefficient of the sample.
Alternatively, the broadband light may be sent to the optical probe head without optical filtering and the spectral components at different wavelengths in the output light from the probe head may be selected and measured to measure the response of the sample around a selected wavelength or the spectral distribution of the responses of the sample. In one implementation, a tunable optical bandpass filter may be inserted in the optical path of the output light from the probe head to filter the light. In another implementation, a grating or other diffractive optical element may be used to optically separate different spectral components in the output light to be measured by the detection subsystem or the detector.
As an example,
Notably, the devices and techniques of this application may be used to select a layer within a sample to measure by properly processing the measured data. Referring back to the devices in
In operation, the following steps may be performed. First, the differential delay modulator 250 is adjusted so that the path length traveled by one mode (e.g., the mode 001) matches that of radiation reflected from interface I in the other mode (e.g., the mode 002). At this point, the pass band of filter 1610 or 1710 may be scanned while recording the oscillation of the measured signal due to a periodic differential phase generated by the modulator 250. The oscillation amplitude as a function of wavelength is given by
where zI is the distance of interface I measured from the top surface of the sample 205. Next, the differential delay modulator 250 is adjusted again to change the differential delay so that the path length traveled by the mode 001 matches that of radiation reflected from interface II in the mode 002. The measurement for the interface II is obtained as follows:
where zII is the distance of interface II measured from interface I. To acquire the absorption characteristics of the layer bounded by the interfaces I and II, Eq. (7) and Eq. (6) can be used to obtain the following ratio:
Notably, this equation provides the information on the absorption characteristics of the layer of interest only and this allows measurement on the layer. This method thus provides a “coherence gating” mechanism to optically acquire the absorbance spectrum of a particular and designated layer beneath a sample surface.
It should be noted that the pass band of the optical filter 1610 or 1710 may be designed to be sufficiently narrow to resolve the absorption characteristics of interest and at the meantime broad enough to differentiate the layer of interest. The following example for monitoring the glucose level by optically probing a patient's skin shows that this arrangement is reasonable and practical.
Various dependable glucose monitors rely on taking blood samples from diabetes patients. Repeated pricking of skin can cause considerable discomfort to patients. It is therefore desirable to monitor the glucose level in a noninvasive manner. It is well known that glucose in blood possesses “signature” optical absorption peaks in a near-infrared (NIR) wavelength range. It is also appreciated the main obstacle in noninvasive monitoring of glucose is due to the fact that a probing light beam interacts, in its path, with various types of tissues and substances which possess overlapping absorption bands. Extracting the signature glucose peaks amongst all other peaks has proven difficult.
The above “coherence gating” may be used overcome the difficulty in other methods for monitoring glucose. For glucose monitoring, the designated layer may be the dermis layer where glucose is concentrated in a network of blood vessels and interstitial fluid.
Therefore, the coherence gating implemented with the devices in
It is clear from Eq. (18) that the product of spectral resolution and layer resolution is a constant for a given center wavelength λ0. The choice of the filter bandwidth should be made based on the tradeoff between these two resolutions against the specific requirements of the measurement.
The tunable bandpass filter 1610 or 1710 may be operated to acquire the absorption characteristics of an isolated volume inside a sample.
In operation, each detector element receives light in a small wavelength interval. The photocurrents from all elements in an array can be summed to form a signal which is equivalent to the signal received in each single detector without the grating shown in
Only a few implementations are disclosed in this application. However, it is understood that variations and enhancements may be made.